The compatibility between Cu and liquid Li directly affects the safety of fusion devices. In this work, we investigated the compatibility between Cu Sigma 3 (111), Sigma 3 (112) and Sigma 5 (310) symmetrical tilt grain boundaries (STGB) and liquid Li using molecular dynamics (MD) simulation. The Li atoms rapidly penetrate the intergranular area along Sigma 3 (112) and Sigma 5 (310) grain boundaries (GBs), resulting in the formation of liquid grooves at the junctions of GBs and solid-liquid interfaces. However, the Sigma 3 (111) GB shows its resistance to Li penetration. In addition, the interfacial alloying and dissolution of Cu atoms are accelerated by Sigma 5 (310) GB via promoting the escape of Cu atoms. Then we calculated the potential energy distribution of Cu atoms which indicates the liquid metal embrittlement (LME) effect exists between Cu bicrystal and liquid Li. And we demonstrate this embrittlement has a strong correlation with the vacancy formation energy and the segregation energy of Li atoms. Comparing with the experimental results, our simulation results explain the corrosion morphology of polycrystalline Cu in liquid Li, and point out that the LME is the root cause of Cu specimen fragmentation. (C) 2020 Elsevier B.V. All rights reserved.